Power generation by inertial confinement fusion using high power, short pulse lasers has led to the development of very large high gain laser amplifiers. These amplifiers generate high power laser beams which are focused onto a small nuclear fuel pellet to produce fuel compression and burn. The gain medium and the complex optics associated with these amplifiers introduce wavefront distortions into the high power laser beam resulting in reduction of energy densities on the pellet and uncertainties in the aim of the beam.
One method for providing a corrected beam which might be adapted for use with high power laser amplifiers is that described in U.S. Pat. No. 3,731,103 for "Adaptive Arrays" or U.S. Pat. No. 3,967,899 for "Method and Apparatus for Maintaining Far Field Spatial Coherency in Electromagnetic Transmitting Systems," both by T. R. O'Meara. These systems incorporate adaptive ptics which utilize coherent optical adaptive techniques to accomplish the wavefront correction. In particular, the systems use a mechanically deformed mirror and a multichannel electro-mechanical serve system to accomplish the desired wavefront correction. Generally, however, these systems are very complex and response times are on the order of one millisecond or longer. Such long response times are generally not useful in inertial confinement fusion systems.
A second method of providing for wavefront corrections is described in U.S. Pat. No. 4,005, 935 for "Method and Apparatus for Providing a Phase Compensated Optical Beam," by V. Wang. This patent describes a nonlinear phase conjugation system which eliminates the need for complex servo systems, and in addition is essentially instantaneous in its response time. However, use of this system, as well as the adaptive optical systems described above requires that a high power laser beam traverse the system components three times in order that a finally corrected beam be obtained.
For the purposes of inertial confinement fusion, however, only a single high power laser beam pulse is allowed to strike the target. This limitation is imposed in order to avoid gain depletion in the laser amplifier and damage to optical components.
Thus, presently existing techniques are inappropriate for use with high power laser amplification systems.